Abstract

Replacing natural habitats with artificial structures such as pier-pilings, jetties,
and seawalls has important consequences to abundances of biota. It is, however, not often
known whether these are direct (the novel habitat alters abundances of some species) or
indirect (the novel habitat directly alters some aspect of the behavior or ecology of some
species, which, in turn, alter abundances of other species). Marine animals in some modified
habitats in Sydney Harbour provide experimental opportunities to test hypotheses to
distinguish between direct and indirect processes. Covers of bryozoans and hydroids were
greater on kelp growing on pilings than on kelp growing on natural reefs. The epifauna may be
affected directly by the pilings or indirectly, i.e., the structure affects characteristics of the kelp
which, in turn, influence covers of epifauna. Thus, differences in covers of epifauna on kelp
can be due to: (1) factors associated with the primary habitats (pilings vs. reefs), (2) differences
between characteristics of the kelp found in each habitat, or (3) an interaction between these
factors (habitat and/or type of kelp). Kelp were experimentally transplanted between pilings
and reefs, demonstrating that properties of the habitat directly affected covers of epifauna,
which were not influenced by the type of kelp that grows on pilings or rocky reefs.
Manipulative experiments to unconfound multiple components of habitats influencing
disturbances to biota are needed to understand human impacts on natural systems.